Many articles of manufacture are termed by densification of fine powders, such as radomes, laser windows, battery electrolytes, cutting tools, penetrators, electrooptical devices, fuel elements, and ceramic turbine blades. It is known that volatile additives can be employed to promote the densification by sintering of fine powders. For example, Atlas in U.S. Pat. No. 2,823,134 teaches the use of lithium compounds in the sintering of cold-pressed magnesium oxide powder. During heating, these compounds are believed to form a liquid thereby increasing the rate of sintering of the powder while at higher temperatures they are believed to evaporate producing a relatively pure, high density article. Although an object of the Atlas invention was to provide an improved method for densifying magnesium oxide, Atlas was not able to obtain nonporous articles by his method. Rice et al in U.S. Pat. No. 3,301,781 teach the use of lithium halide salts in the sintering of cold-pressed magnesium fluoride. During heating, these compounds are also thought to form a liquid. Rice also was not able to obtain pore-free articles. One difficulty in both the Altas and Rice et al processes is entrapment of insoluble gases by the volatile additive liquid when it melts and flows throughout the compacted powder. Another difficulty is the evaporation of the additive from the powder before all the benefits are obtained.
It is also known that atmosphere control can be employed to enhance the effect of volatile additives. Snow Reports ["Fabrication of Transparent Electrooptic PLZT Ceramics by Atmosphere Sintering", J. of Amer. Ceram. Soc., 56(2) pp. 91-96, 1973; "Improvements in Atmosphere Sintering of Transparent PLZT Ceramics", J. of Amer. Ceram. Soc., 56(9) pp. 479-480, 1973] that large nonporous plates of lanthanum-modified lead zirconate-titanate (PLZT) can be produced by sintering powders with excess PbO present as a volatile additive. To produce nonporous plates, the powders are first cold-pressed to form slugs, then annealed in oxygen in a platinum crucible and finally annealed in an alumina crucible containing PbZrO.sub.3 powder to provide a PbO-rich atmosphere. In Snow's process, oxygen, which is soluble in the liquid, is entrapped by the liquid as it flows throughout the body. The oxygen can escape from the pores by diffusion, allowing the body to shrink. Also, containment of the cold-pressed slugs in the platinum crucible decreases the rate of loss of the PbO-rich liquid by evaporation and thus allows the full benefits of the liquid to be obtained. A goal in Snow's work was to prepare transparent and thus pore-free plates of PLZT. Although this goal was apparently realized, there are several difficulties in applying Snow's process to a broader range of materials. One difficulty is the limited opportunity for outgassing the power inherent in the process. Another difficulty is the use of the soluble gas during the initial annealing treament. It may not be possible to obtain a suitable gas in many systems. Still another difficulty is the continuous loss of the volatile additive during the Snow process. Although containment in the crucible reduces this rate of loss sufficiently in the case of the PbO-rich liquid, it might not reduce it sufficiently in the case of more volatile additives for the full benefits of the liquid to be obtained.
In addition, it is known that volatile additives can be employed to promote the densification of time powders by hot-pressing. Carnall, Jr., in U.S. Pat. No. 3,476,690 teaches the use of LiF as an additive in the hot-pressing of MgO to form pore-free, optically useful elements transparent to both visible and infrared radiation. Similarly, transparent Y.sub.2 O.sub.3 and MgAl.sub.2 O.sub.4 have been obtained by hot pressing with LiF. The use of volatile additives in hot pressing has been proposed in other systems including the densification of CaO, Al.sub.2 O.sub.3, and BaTiO.sub.3. Also, NaF has been proposed as a volatile additive in the hot-pressing of MgO. Although enhanced densification has been observed with these systems, it is not clear that one could obtain nonporous bodies in all cases. One difficulty is the limited opportunity for outgassing the powder due to its containment in the hot-pressing die. Also, hot pressing is generally considered to be a more expensive process for making articles than is cold-pressing and sintering.
In addition to the foregoing deficiencies, it has not been an objective in the previous work to make articles with inter-connected channels in a controlled manner or to make articles with controlled amounts of inter-connected porosity, or to make articles with other than simple shapes. Such articles would be highly advantageous, for example as high surface area cylindrical supports for platinum or palladium catalysts, or as shaped ceramic filters for use in such apparatus as soxhlet extractors.
The present invention overcomes the foregoing drawbacks and provides an inexpensive process and apparatus for densifying a polycrystalline article formed from an inorganic compound in fine powder form. The process is capable of producing substantially nonporous articles from a wide variety of inorganic compounds or it can be operated to produce an article having a controlled amount of interconnected porosity. The apparatus provided herein enables the process to be conducted in a rather simple manner with sufficient flexibility to permit the fabrication of shaped articles having nonporous or controlled porosity characteristics.
Specifically, the process provided herein applies to the densification of a polycrystalline article formed from an inorganic compound in fine powder form in which the powder is mixed with a sintering aid, the mixture is compacted to a predetermined shape and then sintered. The process provides an improvement according to which prior to the sintering step, the compact is heated and subjected to temperature and pressure conditions to outgas the compact but the conditions are insufficient to form a liquid of the sintering aid. The outgassed compact is then subjected to a permeation anneal step in which it is further heated in a closed chamber under temperature and pressure conditions which serve to inhibit evaporation of the sintering aid, for a time sufficient for the liquid to permeate and substantially densify the compact. Thereafter, the sintering aid is removed from the densified compact.
In one embodiment the sintering aid is removed by subjecting the permeated compact to an evaporation anneal step by heating under pressure conditions which permit the sintering aid to evaporate whereby to obtain a substantially nonporous article. In another embodiment, the sintering aid is removed by leaching it from the densified compact with an appropriate liquid solvent at a suitable temperature whereby to obtain an article having a substantial quantity of interconnected pores. The basic features of the liquid solvent include both limited solubility and wettability for the polycrystalline material and at least moderate solubility for the sintering aid.
After the permeation anneal step, but prior to removing the sintering aid, the densified compact can be forged to a desired shape, and providing that temperature and pressure conditions do not permit evaporation of the sintering aid, an article having that shape can be produced with controlled interconnected porosity by thereafter leaching out the sintering aid. Alternatively, the compact can be subjected to conditions which evaporate or solvent-extract the sintering aid to further densify the compact, producing a substantially nonporous article of that shape.
The specific technique used in the present invention to accomplish the foregoing involves conducting the permeation anneal in a closed chamber in which there has been placed a quantity of "atmosphere" material, having a volatility at least as high as the sintering aid to provide an overpressure. The atmosphere material may be of the same composition as the sintering aid in which case the apparatus is subjected to temperature differentials to heat the atmosphere material at a higher temperature than the compacted sintering aid during the permeation anneal thereby inhibiting evaporation of the sintering aid. During the evaporation anneal step, the compact is heated at a higher temperature than is the atmosphere powder, or the compact is heated exposed to the ambient atmosphere, to evaporate the sintering aid causing further densification to a nonporous state.
The apparatus used in conducting the foregoing process comprises a vessel having a cavity with means in the cavity for defining a reservoir section containing atmosphere material and a specimen section in communication with each other, and seal means for containing a liquid seal for the cavity. The seal means can comprise a channel formed through the apparatus from the outside to the reservoir section, opening into that section adjacent the bottom thereof. In one embodiment, the cavity sections are defined in horizontally spaced relation and a wall is provided between the sections defining a passage thereacross. Tight fitting caps close the sections and are removable for purposes of loading the apparatus. The channel is formed through the cap closing the reservoir section. In another embodiment, the specimen and reservoir sections are defined as top and bottom sections, respectively, in vertically spaced relation between a top wall and a bottom wall and a pedestal is provided to support the specimen spaced from the bottom wall of a cavity. In this embodiment, the top wall can be formed with a cap resting on the top edge of the apparatus and which is removable from the apparatus. The top edge is formed with a channel constituting the seal means and the lid is formed with a lip fitting in the channel.
The process of this invention is capable of forming transparent articles from such compacted compounds as magnesium oxide or magnesium fluoride. Particularly with respect to magnesium fluoride, a novel article of manufacture can be produced. Polycrystalline magnesium fluoride windows are transparent to infrared but those currently available are not uniformly transparent throughout the entire region of the infrared spectra, exhibiting one or more undesirable absorption bands at 2.7, 3.0, 5.0, 6.2 or 6.7 microns, that are detrimental to its use as windows, for example for chemical lasers. By practicing the present invention, a body of magnesium fluoride can be produced having a porosity of less than 0.1 volume percent, having a grain size of less than 10 microns and which is substantially uniformly transparent to infrared radiation throughout the entire range of 0.7-8 microns.